Apply Maven POM best practices: pluginManagement, properties, enforcer

Apply Maven POM best practices: pluginManagement, properties, enforcer

- properties: add reporting.outputEncoding, project.build.outputTimestamp
  (reproducible builds); extract all plugin versions as properties with
  standard {artifactId}.version naming; add maven-fluido-skin.version
  (referenced from site.xml via filtering, so stays in sync)
- pluginManagement: declare explicit versions for all Maven lifecycle
  plugins (clean, resources, compiler, jar, install, deploy) — overrides
  outdated oss-parent:9 defaults — plus enforcer and site plugin with
  their full configuration (moves config out of <plugins> into mgmt)
- maven-enforcer-plugin: require Maven >= 3.8.5 (no Java version rule
  since the project has no Java sources)
- maven-resources-plugin: upgrade 2.7 → 3.3.1; remove explicit
  maven-filtering:1.3 dependency (no longer needed in 3.x); rename
  execution id copy-resources → filter-site-resources for clarity
- site.xml: use ${maven-fluido-skin.version} property (filtered at
  pre-site phase, single source of truth in pom.xml properties)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>